The element nitrogen (N) is essential to all living organisms, and the availability of fixed forms of nitrogen is often a limiting factor in food production. Biological nitrogen fixation, which occurs in a diverse group of microbes, represents the single largest input of fixed nitrogen in the reductive phase of the global nitrogen cycle. A molecular understanding of the enzyme responsible for biological nitrogen fixation, nitrogenase, could contribute to enhanced food production worldwide, and thus to the overall health of the human population. In addition, as a representative of the diverse family of metalloproteins and nucleotide-dependent enzymes, understanding the mechanism of nitrogenase will contribute to a broader understanding of these important enzymes. Our goal is to use a comprehensive approach to address some of the significant remaining questions about the nitrogenase mechanism. The related mechanistic issues that will be examined include the following: (i) understanding how MgATP hydrolysis is coupled to substrate reduction; (ii) defining how electron transfer through nitrogenase is controlled; (iii) understanding the roles of the associated metal clusters in this process; and (iv) defining how substrates access and products exit the active site. These goals will be accomplished by use of a wide array of methods including genetic selections, site-directed mutagenesis, purification of nitrogenase proteins with individual and multiple amino acid substitutions, steady state and pre-steady-state kinetics, electrochemistry, and spectroscopic methods including EPR, ENDOR, and X-ray crystallography.